The constructions for fuel cells and/or fuel tanks for aircraft and other vehicles that can be exposed to ballistic projectiles in use has basically remained the same for many years. The fuel tank constructions and the materials have not changed in over 50 years because the existing technology is easily qualified through similarity, and because a robust full cell industrial base existed for producing the fuel tanks. However, over the last decade the industrial base has begun to shrink. It has also become more difficult to fabricate fuel tanks employing the accustomed materials and methods of construction due to environmental regulations that have made many of the materials used to fabricate the fuel tanks obsolete.
The conventional construction of crash-worthy and ballistic tolerant fuel tanks has relied on a non-permeable fuel cell membrane to contain the fuel, a self-sealing element to seal the fuel tank should a ballistic projectile produce an opening or hole in the tank, and outer reinforcement plies.
The outer reinforcement plies are typically nylon reinforced vulcanized nitrile-rubber. These reinforcement plies are designed with sufficient strength to overcome the forces generated from a 65 foot fluid filled drop test of the tank and a ballistic hydrodynamic event. Overcoming these forces makes this type of construction of the fuel tank inherently heavy.
The self-sealing element of the conventional fuel tank is typically provided by natural gum rubber. Crash-worthy and ballistic tolerant fuel tanks are provided with a self-sealing element based on the chemical reaction that occurs when the natural gum rubber employed in fabricating the tank is exposed to the hydrocarbons in fuel released from the tank due to an opening or hole produced by a ballistic projectile. This interdependency between the self-sealing material of the fuel tank and the fuel type held by the fuel tank that initiates the self-sealing functioning is a constraint on the operation and design of current ballistic tolerant and self-sealing fuel systems.